In the United States there appears to be an ever increasing need to reduce the depletion of our forests by increasing the use of steel or structural plastics. This invention pertains to the design and manufacture of steel roof trusses on a most economical basis, taking into account total pounds of metal, assembly labor, and shipping costs.
With high speed computers it is now possible to design lowest cost trusses taking diverse factors into consideration for each job. This invention takes advantage of such computer capabilities.
Minimum metal usage will be achieved using minimum pounds of metal for shapes and reinforcing arrangements that give needed strength to meet the structural codes (which have, of course, necessary safety factors) but a most economical truss means most economical installed truss and as such, material handling, shipping, truss spacing to not only carry the usual live and dead loads but, in some cases, to give necessary between--truss space, must all be, taken into account along with assembly time and labor cost.
Roll forming of flat metal to form truss member shapes is an economical method of manufacture. Larger machines now allow rolling of U shaped members with more than 5" legs. In a preferred embodiment of the invention the top chord truss members are rolled in a flat bottomed U shape with one or more reinforcing ridges rolled into the base of the U that may be more than 1" wide and with one or more reinforcing ridges in each of the legs of the U that may be more than 5" in length and finally with a rolled out flange at the top of the legs--overall shape may be easily roll formed and gives near maximum strength for pounds of metal used. Since handling of very long lengths is difficult, the top chord and also bottom chord members are usually made approximately 20' long. Now spans of more than 70' are frequently required and shipment of a 70' truss with the usual 5 in 12 pitch becomes expensive because of size and difficulty of handling such a large unit. Thus, in many cases, job site assembly of individual trusses is preferred. Shipment of truss members up to 20' in length is quite economical. The invention encompasses specially rolled-to-fit splicing sections for both top and bottom chord members and special heel reinforcing sections for load bearing joints formed from segments of web members of the truss. These segments may be about 6" long and installed in vertical position in the load bearing joint formed where the bottom chord of the truss ties into the top chord near a terminal end of each truss unit.
The bottom chord members have a U shaped cross section with one or more reinforcing ridges rolled into the flat bottom of the U shape and with one or more reinforcing ribs in the legs of the U, which may be more than 3", and with the ends of the legs folded back to form a double thick edge.- With the shape as described the end of the lower chord member will slide into the end of the upper chord member to form a load bearing joint at each end of the lower chord member. As previously described a vertical segment of a C shape web member is used in the joint for further reinforcement.
With lower chord members usually manufactured in 20" lengths a second rolled to fit splicing section that may be more than 12" long is used to splice butted-together ends of chord members to form a chord unit.
Truss web members have a C shaped cross section with one or more reinforcing ribs in the side of the C shape and are sized to fit easily into and between the upper and lower chord units.
Power driven self tapping screws are used in one preferred embodiment for assembly. Bolts or rivets could also be used or a combination of different fastening methods could be used for assembly.
The computer design program checks for most economical design using 14, 16, 18, or 20 gauge metal and using heavier gauge metal members to reinforce a weaker point in the truss design. Final design also indicates the number and size of self tapping screws at each connecting point in the truss in a preferred embodiment.
We have considered the following patents in Class 52, subclasses 634, 639, 641, 643, 690, 692 and 693:
U.S. Pat. No. 1,311,486
U.S. Pat. No. 2,541,784
U.S. Pat. No. 3,656,270
U.S. Pat. No. 4,074,487
U.S. Pat. No. 4,435,940
U.S. Pat. No. 4,416,453
U.S. Pat. No. 4,720,956
In our opinion the subject invention is uniquely different to fill the need for the lowest cost truss for various structural requirements.